Heat exchange – Casing or tank enclosed conduit assembly – Manifold formed by casing section and tube sheet of assembly
Patent
1996-09-16
2000-06-06
Leo, Leonard
Heat exchange
Casing or tank enclosed conduit assembly
Manifold formed by casing section and tube sheet of assembly
165179, 165183, 138 38, 138157, F28F 140, F28F 142
Patent
active
060706570
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a heat exchanger tube for heating boilers, particularly for gas condensing boilers, in accordance with the precharacterising portion of claim 1.
In condensing boilers, which are principally gas fired heating boilers, the combustion gases are cooled until the exhaust gas moisture condenses in order to utilise the heat of condensation. The prerequisite for this is that the heating boiler is operated with a boiler water temperature which is lower at the end of the combustion gas path through the heating boiler than the dew point temperature of the combustion gases. One endeavours to cool the combustion gases over as short as possible a path of the combustion gases through the water cooled heat exchanger tubes of the heating boiler from the high inlet temperature, which can be about 850.degree. C. with modern gas burners, to a temperature which is between the dew point temperature and the lowest boiler water temperature of e.g. 30.degree. C. prevailing at the heated water return. Heat exchanger tubes are known for this purpose which comprise a cylindrical, smooth walled outer tube of steel which is resistant to acid corrosion by the exhaust gas condensate and an aluminium profiled insert of star-shaped cross-section pushed into the outer tube. For heating boilers of the most usual construction the outer tube must comprise steel in order to be able to be welded at its ends into tube bases or tube plates which separate the boiler water space surrounding the heat exchanger tubes from the combustion chamber on the one hand and from the exhaust gas manifold of the heating boiler on the other hand. The composite tube consisting of the steel outer tube and aluminium profiled insert can be subjected to high inlet gas temperatures because aluminium has a larger coefficient of expansion than steel so that the profiled insert remains in thermally conductive contact with the outer tube at its contact points with the external tube with a pressure which actually increases with increasing temperature. In the known composite tube the transfer of heat from the star-shaped aluminium profiled insert to the steel outer tube is determined and limited by the fact that the profiled insert contacts the outer tube only at the ridge surfaces of the radiating arms of the profiled insert which are relatively thin walled in cross-section in order to leave a sufficient area free in the outer tube for the flow of combustion gas. It has also proved to be necessary for the welding of the steel outer tube into the tube plates that at the ends of the outer tube the ends of the star-shaped aluminium profiled insert must be sufficiently set back in order to prevent the radiating arms of the aluminium profiled insert being destroyed by the welding heat produced at the outer tube ends.
The invention has the object of providing a heat exchanger tube of the type referred to above which makes an even greater heat transfer capacity possible from the combustion gases to the boiler water and can be simply manufactured and further processed when being installed in a heating boiler. The invention solves this object by the construction of the heat exchanger tube constituted by a composite tube of a steel outer tube and an aluminium profiled insert with the characterising features of claim 1.
The tubular body-shaped profiled insert of the heat exchanger tube in accordance with the invention can be constructed on the one hand with a very large internal surface area which receives heat from the combustion gases, preferably with ribs disposed in the manner of a comb on the internal surface of the two half shells, and, above all, engages the inner surface of the water cooled steel outer tube with a substantially larger outer surface area in comparison to the known star profiles, whereby the heat transfer capacity from the combustion gases to the boiler water is significantly increased. It has been determined in experiments that with a condensing boiler, in which the returned heating water has a water temperature of about 30.
REFERENCES:
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patent: 1350073 (1920-08-01), Dow
patent: 1692529 (1928-11-01), Zagorski
patent: 2618738 (1952-11-01), Foulds
patent: 3267563 (1966-08-01), Seaton
patent: 3267564 (1966-08-01), Keyes
patent: 3870081 (1975-03-01), Kleppe et al.
patent: 4899813 (1990-02-01), Menicatti et al.
Hoval Interliz AG
Leo Leonard
LandOfFree
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